Portable radio systems are sometimes used in potentially explosive atmospheres. A portable radio system used in potentially explosive atmospheres should be designed to meet the safety standards of Directive 94/9/EC of the European Parliament and the Council of the European Union, commonly referred to as ATEX, which is derived from the French words ATmosphères EXplosibles. A portable radio system that meets the ATEX standards may be referred to as having an “intrinsically safe” rating. A portable radio system that has an intrinsically-safe rating is less likely to ignite an explosive atmosphere while operating within such an environment.
FIG. 1 is a cut-away, perspective view 100 of a simplified diagram of a known portable radio system 101. The known portable radio system 101 includes a known radio device 102 and a known battery pack 104. The known battery pack 104 includes a battery 106 and a current-limiting protection circuit 108. The known battery pack 104 has an interface system 110 for coupling with a complementary interface system (not shown) on the known radio device 102. The interface system 110 comprises a logic contact 111, and direct current (DC) power supply contacts including a positive contact 112 and a negative contact 114. The positive contact 112 and the negative contact 114 are connected, within the known battery pack 104, to positive and negative terminals, respectively, of the battery 106. The known radio device 102 also includes a display 122, a keypad 124, a channel select knob 126, a volume control knob 128, an antenna 132, an audio power amplifier 146, a speaker 148, a radio frequency power amplifier 156 and other electronics (not shown).
With an intrinsically-safe radio system, any short circuit electrical energy exposed to an explosive atmosphere should not cause ignition. Sparking at the DC power supply contacts should be limited to a sufficiently low energy so that the explosive atmosphere will not ignite even when component faults and/or short circuits exist in the known portable radio system 101. Sparking at the DC power supply contacts of the interface system 110 can occur if the known portable radio system 101 is dropped in an explosive environment, or when the known radio device 102 and the known battery pack 104 are disconnected or re-connected from each other in an explosive environment. A sparking current as low as 1.5-amps can have enough energy to ignite an explosive atmosphere.
A known method of limiting short circuit electrical energy utilizes a fast current-limiting protection circuit 108 to limit current to less than approximately 1-amp under contact short circuit conditions, thereby reducing the probability of occurrence of an igniting energy “let through”. The phrase “contact short circuit” means any current path between the contacts that may result in sufficient energy to cause an ignition. The term “let through” means the amount of energy that is available at the contacts before the current-limiting protection circuit can interrupt or reduce the contact current. However, there is at least one disadvantage to circuits that limit igniting energy. To sufficiently limit available short circuit igniting energy, a current limiting threshold should be relatively low and also extremely fast. However, a normally operating known radio device 102 may demand a high peak DC current from the battery pack 104 for proper functioning of a radio transmitter. Typically, a transmitter within the radio device 102 requires 1.0-amp to 1.8-amp, depending upon capacitance. The let through energy limitations for preventing ignition, and the let through energy capability for fully supporting normal radio transmitting, may overlap and, as a result, be troublesome. This overlap becomes more troublesome when practical complex circuit tolerances are taken into account. The conflicting requirements of energy limitations for preventing ignition, and energy capability for fully supporting normal radio transmitting, result in compromises in radio design performance, including limiting transmitter output RF power to undesirably low levels. A current-limiting protection circuit should have a low energy trip threshold to prevent ignition. The current-limiting protection circuit should have a high energy trip threshold to support radio transmissions under full power. The performance of a portable radio system may be degraded in an attempt to prevent overlap of these two requirements.
A known solution to this problem is making the circuit tolerances as small as possible, and optimizing the radio circuit current demand to be as low as possible, especially under transmit peak power situations. However, even with these efforts, the let through energy limitations to prevent ignition, and the let through energy capability to fully support normal radio transmitting overlap, and, as a result, sub-optimal performance occurs.
FIG. 2 is a simplified function block diagram 200 of the known portable radio system 101. The known radio device 102 comprises a receiver 140 and a transmitter 150. The receiver 140 and the transmitter 150 are coupled to a transmit/receive switch 134. The transmit/receive switch 134 is coupled to the antenna 132. The other electronics 160 of the known radio device 102 includes receiver circuits 142, an audio synthesis audio driver 144, a carrier synthesis circuit 152, a modulator and radio frequency driver 154, a microprocessor 170, circuits 172 associated with the display 122, circuits 174 associated with the keypad 124, and circuits 176 associated with accessories (not shown). In FIG. 2, the positive contact 112 and the negative contact 114 of the known battery pack 104 are shown coupled to a positive contact 182 and a negative contact 184, respectively, of the known radio device 102.
Therefore a need exists to overcome the problems with the prior art as discussed above.